This is a pretty cool feature that I heard about that is present (well, not working, but the Control Panel applet is there) in Windows Vista build 5308. This will basically allow you to upgrade your version of Windows Vista to a better edition (the finally announced product editions are here), if you decide later that you want the extra features (e.g. the Media Centre features in Vista Home Premium).
From Home Basic you can upgrade to Home Premium or Ultimate Edition as seen here:
The interesting thing about this is that the functionality for all the versions of Windows must be on one disc (note that you only download a 'licence' to upgrade, not a big load of files). That must mean there is a great potential for people to crack open copies of Windows Vista Home Basic and turn them into Ultimate Edition.
(Screenshot courtesy of Paul Thurrott's SuperSite for Windows)